Method and apparatus for washing pulp



1968 o. SEPALL METHOD AND APPARATUS FOR WASHING PULP Filed Oct. 12, 1964 PATENT AGENT United States Patent 3,419,466 METHOD AND APPARATUS FOR WASHING PUkP t A glo Ola Se all Quebec, Quebec, Cana a, asslgnor 0 n Pap r Iroducts Limited, Quebec, Quebec, Canada Filed Oct. 12, 1964, Ser. No. 403,004 2 Claims. (Cl. 162-204) ABSTRACT OF THE DISCLOSURE A method and apparatus for washing pulp wherein the pulp is formed into a sheet on a traveling screen, the improvement which comprises mov ng the screen with the pulp sheet thereon through a continuous body of wash liquid to submerge therein a port1on of the screen and pulp sheet while simultaneously creating a difference in pressure across the submerged portlon of the screen and pulp sheet to cause afiow of wash llquld through the screen and sheet.

This invention relates to a method and apparatus for washing pulp.

A common procedure employed in pulp mills to wash black liquor from brown pulp stock involves the use of multistage rotary drum vacuum or pressure washers.

The present invention relates to thls type of washing procedure but has for an object, the provision of a method and apparatus which results in greater capacity, less foaming during the procedure, higher discharge consistency, the use of higher vacuum, and a general increase in washing efficiency.

Conventional washers have several disadvantages and are rather inefficient for chemical recovery because recovered liquors are extensively diluted and so larger evaporating capacity and steam consumption are needed in the recovery system. Moreover, with these washers, it has not been possible to avoid the entry of air into the system resulting in a serious foam problem in many mills. Cost of defoaming chemicals is a substantial expense item in many kraft mills. Furthermore, the capital cost of a washing installation is large because a separate washer is required for each stage of which there are usually three or four.

In the conventional washer, a pulp suspension enters the washer vat where the liquid drains through the wire leaving a pulp mat on the surface of the rotating cylinder. After the cylinder has emerged from the liquor, washing liquid is sprayed on top of the pulp mat to displace and wash out liquor therein. The washing effect obtained with such sprays has limited elfectiveness since 1) the area of contact is relatively small, (2) the application of wash liquid is not uniform, and (3) displacement of liquor is not obtained without considerable mixing. Obviously, air is entrained in the sprays and enters the sheet, thus producing foam in the filtrate.

The present invention relates to a method of washing pulp including the conventional steps of moving a screen through a body of pulp stock and creating a difference in pressure across the screen to form a pulp sheet thereon. The invention resides in the steps of applying a body of wash liquid to the pulp sheet-covered surface of thescreen following its movement through the stock body whereby the pulp sheet is submerged during its movement through the body of wash liquid, and creating a difference in pres sure across the screen and pulp sheet during movement through the body of wash liquid to cause a flow of wash liquid through the sheet. Following emergence from the body of wash liquid, a differential in pressure may be effected across the screen and pulp sheet to remove excess Patented Dec. 31, 1968 wash liquid therefrom after which the pulp sheet is removed from the screen.

The invention also relates to a pulp washer having a pulp stock receiving vat, and a drum having a wire screen on its cylindrical surface rotatably mounted about a horizontal axis in the vat. The vat has a stock level area, and the drum has an upper portion disposed below said area and a lower portion disposed above said area. A wash liquid compartment is mounted in the vat and has at least a portion disposed below the stock level area, such compartment extending substantially from end to end of the drum. The compartment has a wall in confronting relation to a longitudinally extending section of the lower portion of the drum, the screen in such section being exposed to the interior of the compartment. The compartment wall has a top edge disposed above the stock level area and a sealing strip constituting its bottom edge sealingly presses against the screen. The compartment is provided with end Walls sealingly separating it from the interior of the vat. Means are provided for selectively creating a difference in pressure through a plurality of longitudinal sections of the screen.

The invention will be further described with reference to the accompanying drawing, in which:

FIGURE 1 is a perspective end view, partly in section, of an apparatus in accordance with the invention,

FIGURE 2 is a diagrammatic end view of the drum,

FIGURE 3 is a partial plan view of the wash liquid, compartment, and

FIGURE 4 is a sectional of apparatus.

Referring to FIGURES 1, 2 and 3 of the drawing, 10 1s a vat adapted to contain a body of pulp stock 11 as fed from a stock inlet section 12.

A cylindrical drum 13 is mounted in the vat for rotation about its axis, which is horizontally disposed, as by means of a gear 14. The cylindrical surface of the drum is covered with a woven wire screen 15. The drum structure, as well as its mounting and driving means, is of conventional type, and includes usual means for drawing vacuum from within the drum. The drum is partially submerged in the body of pulp stock and, as shown, the stock level area, indicated generally at 16, may be disposed at or somewhat above the hemispherical level of the drum.

In the example shown, it is contemplated that vacuum will be separately drawn from two separate sections of the drum surface, namely; (as shown in FIGURE 1) Section B, during the travel of the drum through at least a portion of the pulp stock-engaging section thereof, and Section A, during the upward travelling section of the drum after it has emerged from the stock body. A minor Section C of the travelling drum surface is not subject to vacuum.

The vacuum connections, which are of conventional structure and arrangement, include a plurality of pipes 17 leading from vacuum area 18, defining Section A, to rotating valve member 19, and a plurality of pipes 20 leading from vacuum area 21, defining Section B, to rotating valve member 19. Stationary valve member 22 has a pipe 23 connected through the valve members and pipes 17 to vacuum area 18, and a pipe 24 connected through the valve members and pipes 20 to vacuum area 21, each pipe 23 and 24 being connected to vacuum pump means.

It will be apparent that, as the drum 13 rotates through Section B, i.e., the section which is immersed in the pulp stock body, suction through the wire 15 will cause formation of a pulp mat or sheet 25 thereon in the usual manner. In accordance with the invention, however, following such mat formation, it is subjected to immersion in one or more successive bodies of wash liquid for drainage by end view of a modified form 3 suction of portions of such bodies through the pulp mat.

Referring to FIGURE 1, a first wash liquid compartment 26 is provided by means of a wall 27 extending from end to end of the drum and end walls 28 each having a sealing connection 29 with the end of the drum. Wall 27 is located in spaced opposed relation to a section of the drum following its movement through the major portion of the stock body whereby a pulp mat has been formed on such section. Wall 27 has a flexible sealing strip 30 extending along its lower edge and forming a sealing connection between wall 27 and pulp mat 25. As shown, strip 30 has an edge portion which rides on the surface of the mat. The strip 30 may be formed of any suitable composition such as rubber, plastic, or the like.

In the modification shown, compartment 26 is disposed below the level of stock body 11.

A second compartment 31 may be provided by applying a fiexible wall 32 to the interior surface of wall 27, such wall 32 having its edge portion riding on the pulp mat and located in spaced opposed relation to a section of the drum following its movement through the compartment 26. Compartment 31, as shown, is located partly below the level of the stock body.

Compartment 26 is arranged to contain a body 33 of wash liquid, and compartment 31 is arranged to contain a body 34 of wash liquid. A distribution manifold 35, desirably connected to a source of fresh wash water, feeds compartment 31, and a distribution manifold 36, desirably connected to pipe 23 and thus arranged to employ the filtrate from Section A as the wash medium in compartment 26, feeds the latter compartment.

In the modification shown, it will be observed that the pulp sheet, during its entire formation and washing is not exposed to air, i.e., it is substantially continuously submerged. It will be understood, however, that there would be no substantial disadvantage in arranging one or both compartments 26 and 31 above the pulp stock level.

An essential feature of the present invention resides in the fact that the washing step or steps is conducted while the pulp sheet is immersed whereby a much more efficient and uniform washing procedure is effected with no possibility of air entrainment.

The pulp sheet is taken off the drum in Section C in the usual manner as by the use of take-off rolls 37 and 38.

Although in the above example a washer is employed which operates by application of vacuum to the interior of the screen-supporting drum, it will be understood that the invention may be quite as effectively applied to a washer wherein pressure is applied to the external surface of the screen to force liquor through the screen.

Such a washer is illustrated, by way of example, in FIGURE 4, wherein a vat 39 contains a body of pulp stock 40 as fed from a stock inlet section 41.

A cylindrical drum 42 having a screen 43 thereon is mounted in the vat for rotation about its axis. Two wash liquid compartments 45 and 44, corresponding to compartments 26 and 31, are provided. Compartment 45 contains a body 45:: of wash liquid fed by a distribution manifold 46. Compartment 44 contains a body 47 of wash liquid fed by a distribution manifold 48. Filtrate from each body of wash liquid flows into an appropriate 4 drum sector 49, 50 and thence through outlet pipes 51, 52.

The vat and exposed drum surfaces are enclosed by means of a wall 53 having a sealing strip 54 engaging a sealing roll 55. The vat has a sealing strip 56 engaging a take-off roll 57. The paper web 58 passes through the nip of rolls and 57.

The space in the vat above the surfaces of the bodies of stock and wash liquid is pressurized by means of air or steam fed through pipe 59.

I claim:

1. In a method of washing pulp including the steps of rotating a horizontally disposed drum having a screen on the cylindrical surface thereof in a body of pulp stock and black liquor, and creating a difference in pressure between the outside and inside of said drum to form a pulp sheet on said screen, the improvement which comprises applying a body of fresh wash water to the pulp sheet covered surface of the screen following its movement through said stock body whereby said pulp sheet is continuously submerged during its movement through said bodies of liquor and wash water, creating a difference in pressure across the screen and pulp sheet during movement through said body of wash water to cause a flow of wash water through said sheet, creating a difference in pressure across said screen and pulp sheet following emergence of said body of wash Water to remove said excess wash liquid therefrom, and finally removing said pulp sheet from said screen.

2. In a method of washing pulp including the steps of rotating a horizontally disposed drum having a screen on the cylindrical surface thereof in a body of pulp stock and treating liquor, and creating a difference in pressure across said screen to form a pulp sheet thereon, the improvement which comprises applying a first body of wash liquid to the pulp sheet covered surface of said screen directly following its movement through said stock body, applying a second body of wash liquid to said pulp sheet covered surface of said screen directly following its movement through said first body of wash liquid, said pulp sheet being thereby continuously submerged during its movement through said bodies of liquor and wash liquids, creating a difference in pressure across said screen and said pulp sheet during movement through said bodies of wash liquid to cause a flow of wash liquid through said sheet, creating a difference in pressure across said screen and said pulp sheet following emergence from said second body of wash liquid to remove excess wash liquid therefrom, and finally removing said pulp sheet from said screen, said second body of wash liquid comprising fresh water, and said first body of wash liquid comprising filtrate collected from wash liquid drawn through said screen and pulp sheet from said second body of wash liquid.

References Cited UNITED STATES PATENTS 2,995,187 8/1961 Wells 162-390 3,330,723 7/1967 Nisser l62208 HOWARD R. CAINE, Primary Examiner.

US. Cl. X.R. 

